[Spec][Matrix] Define Matrix trunc as a Dimensionality Reduction

[farzonl]

We should make sure the language spec is up to date with the conversion ranks (see: > https://github.com/microsoft/hlsl-specs/blob/main/specs/language/overloading.tex#L296).

Originally posted by @llvm-beanz in llvm/llvm-project#168915 (comment)

conversions.tex already defines matrix truncation as the same as vector truncations.

We however don't mention matrix trunc in the standard conversion sequences so we should add it